JET carbon screening experiments using methane gas puffing and its relation to intrinsic carbon impurities

Abstract

JET carbon screening experiments were performed using methane gas injection. L-Mode experiments scanned parameters influencing the JET scrape-off-layer (SOL) and/or intrinsic impurity level. Scaling relations are derived to describe methane injected into L-Mode plasmas from the JET horizontal mid-plane. L-Mode screening was 3–20 times better for plasmas connected to the divertor than for similar limited plasmas. The screening was worse for methane injection from the mid-plane and best for injection from the divertor. The screening was 1.5–2 times worse for H-Mode than L-Mode. Both ELM-averaged and inter-ELM H-Mode screening was documented. The screening results were used to understand the intrinsic impurity levels. Zeff reduced at higher densities partly due to better carbon screening at the higher density, and partly due to decreased carbon influxes. Diverted L-Mode intrinsic carbon levels arose from both main chamber and divertor sources, while H-mode carbon primarily originated from the divertor. DIVIMP and EDGE2D were used to model the observed screening. The modelling indicated that carbon removal to the divertor required lower temperatures for Coulomb collisions to couple the impurity ions to the SOL deuterium flows. The carbon removal occurred primarily in the outer SOL regions.

DIVIMP modeling of the toroidally symmetrical injection of 13CH4 into the upper SOL of DIII-D

“…Ref. [1] provides an extensive set of references for methane injection experiments including studies on core carbon transport, plume characterization, erosion and re-deposition, molecular break-up studies, and evaluation of carbon penetration through the SOL to the core.…”

Methane Screening in JET Reverse Field Experiments

“…The reverse field screening experiments reproduced normal field JET LMode discharges [1]. However, the Gas Box Divertor (Mk II GB) was used for the normal field experiments, and the Septum Replacement Divertor (Mk II GB SRP) was used for the reverse field experiments.…”

“…However, the Gas Box Divertor (Mk II GB) was used for the normal field experiments, and the Septum Replacement Divertor (Mk II GB SRP) was used for the reverse field experiments. Since the Gas Box carbon screening was insensitive to the divertor configuration [1], the different divertor configuration is assumed to not influence the results. 4 Deuterated methane was puffed at the horizontal mid-plane for about 3 sec.…”

“…Thus the carbon fuelling efficiency was similar for normal and reverse field. The carbon fuelling efficiency [1] is defined as FE = N C / ( C p *).…”

“…Methane screening experiments [1] have been used to benchmark the effectiveness of the SOL and divertor at preventing impurities from contaminating the plasma core. The carbon fuelling efficiency is measured when a known amount of carbon is injected.…”